subclassing Exceptions

[Erik Max Francis]

Sheila King wrote:

> Thanks, Brian, but I've seen the docs many times. I need more info
> than
> that. Like example code, or source code.

The library reference quoted states pretty clearly:

	The root class for exceptions. All built-in exceptions are
	derived from this class. All user-defined exceptions should
	also be derived from this class, but this is not (yet) enforced. 
	The str() function, when applied to an instance of this
	class (or most derived classes) returns the string value of the
	argument or arguments, or an empty string if no
	arguments were given to the constructor. When used as a
	sequence, this accesses the arguments given to the
	constructor (handy for backward compatibility with old code). 
	The arguments are also available on the instance's args
	attribute, as a tuple.

This gives you a pretty effective specification for what a class derived
from exceptions.Exception should do; it should return something
meaningful in its __str__ method, support __len__ and __getitem__ for
indexing, and have an `args' attribute that contains the arguments it
was instantiated with as a tuple.

The Exception class itself does this.  If all you want is a different
_type_ of exception that you can key off of, then deriving from the
class and doing nothing with it (using `pass' as you did in your
example) is perfectly satisfactory.  If you want to do more, then you
can add extra methods as you need that use the value gained through
calling `str' or the `args' attribute.

What more did you need to know?

-- 
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